This invention relates to a sequence control system for a numerically controlled machine tool equipped with a programmable sequence controller.
The operation of machine tools is controlled in order to effect a variety of tasks, such as the automatic change of tools, by combining a numerical control device (referred to hereinafter as an NC) and a programmable sequence controller (referred to hereinafter as a PC). The control of a machine tool through this combination of the NC and PC is carried out in the following manner. Numerical control information stored in a storage medium such as a perforated tape is read in the NC which then processes the information. After all of the necessary machining data for controlling the relative movement between the workpiece and tool, such as feed speed and the numerical values relating to dimensions, has been processed by a pulse distribution operation or the like, the data is sent directly from the NC to the machine tool side, whereas information other than that directly required for the abovementioned control of relative movement, such as information pertaining to the change of tools, is sent to the PC where it is stored. After the PC has determined that the conditions for a tool change on the machine tool side have been established by receiving from the machine tool such signals as a spindle oriented stop signal, the PC sends tool change information to the machine tool which responds to the command from the PC so as to effect the tool change. This entire series of operations is executed in accordance with the program that has been loaded in the PC.
Factories or plants where machine tools are installed do not always possess power distribution facilities that are ideal in nature, and some of them have very poor power source environments. Machine tools installed in locations beset by these problems encounter abnormal conditions such as large fluctuations in power source voltage and interruptions in power, followed by a restoration of normal power source voltage after a short period of time. If a machine tool equipped with a PC and NC encounters a brief interruption in power,the machine tool may, for example, fail to execute an instruction issued by the PC. If the interruption in power occurs during the execution of an instruction, the machine tool may stop functioning in the middle of an operation. Once the power source voltage has returned to normal following restoration of power, the PC, NC and machine tool begin functioning again. When this occurs the NC and PC send new instructions to the machine tool since the machine tool operation instructed prior to the power interruption is taken as being completed. However, this operation instructed before the interruption may never have been executed or may have stopped during execution. Therefore, when the PC and NC issue new instructions following the restoration of power transmission, the machine tool initiates an operation in accordance with the new instructions and leaves the immediately preceding task or operation unfinished. This not only leads to a break in the sequential control of the machine tool but also results in an unsatisfactorily machined workpiece.